The only thing I don't see mentioned in the explanations that already exist is that S3 is a pay for usage service.

I.E. You don't pay for a tier of storage you pay for the gb of storage used. If your usage increases your cost goes up and if your usage goes down your costs go down. Which makes it very easy to set and forget. No need to worry about keeping your data storage under a certain amount or to remember to switch to a lower tier to save money etc.

Granted that can backfire and result in a surprise bill way more than you were expecting but that is usually not an issue especially for medium to large size businesses.